It is a known problem within the circle of persons skilled in the art, that chlorine which is formed in combustion of chlorine-containing fuel leads to corrosion in combustion plants. It is commonly assumed that chlorine is transported in combustion gases in the form of gas phase alkaline chloride (in wood fuel combustion mostly as potassium chloride KCl), alternatively in the form of quite small aerosols of alkaline chloride, to the surface of super heaters and other components that are exposed to the flue-gas in the combustion plant. Reactions between the alkaline chloride and iron oxide, e.g., which may take place on the said surfaces, results in formation of free chlorine which is strongly corrosive in this state.
It is also previously known that addition of sulphur to the fuel may have an effect for reduction of corrosion caused by chlorine. The prior art further includes the addition of sulphur outside the fuel, such as in the flue-gas downstream the proper combustion zone. The purpose of these solutions is to provide reactive sulphur dioxide or sulphur trioxide for sulphating alkaline chloride in the gaseous phase, before the same reaches the surfaces of the combustion plant which are subject to chlorine-induced corrosion. Examples on the latter solutions are found, inter alia, in DE 19849022 and WO 02/059526.
The ability of sulphur to reduce the chlorine content in deposits on heat transferring devices is believed to result from conversion of alkaline chloride to alkaline sulphate in the flue-gas, before reaching the heat transferring devices, according to the sum formula:2KCl+SO2+½O2+H2OK2SO4+2HCl  (1),the equilibrium of which is displaced to the right at flue-gas temperatures below 800° C.
Test measurements have shown that a fairly high concentration of SO2 is required in the flue-gas in order for KCl to be significantly reduced in flue-gas temperatures at about 500-1200° C., which equals the temperature range within which the super-heaters in the most common type of combustion plants operate. For instance, measurements at burning wood chips have shown that an SO2 content corresponding to approx. 50-150 mg S/MJ (Mega Joule) fuel is required in the flue-gas in order to achieve a 50% reduction of the potassium chloride content in the flue-gas ahead of the super heaters. This equals an S/Cl mole ratio in the flue-gas of approx. 3-10. The increase of the sulphur dioxide content in the flue-gas entails negative consequences in the form of increased SO2 emissions from the plant (problems with emission regulations and environmental permits) and an increased risk of low-temperature corrosion. The reason for such a large SO2 redundancy being required is that the reaction (1) is kinetically limited at the flue-gas temperature in question, and has therefore no time to reach equilibrium before the chloride encounters the super heaters.
The reaction (1) is an overall reaction including a plurality of sub-steps. An important sub-step is considered to be:SO2+O(+M)SO3(+M)  (2)where M is a catalyst for the oxidation. The sulphur trioxide formed will further react with the alkaline chloride according to the formula:2KCl+SO3+H2OK2SO4+2HCl  (3)M may, for instance, be iron oxide. The equilibrium of the reaction (2) moves to the right at decreasing temperature. At temperatures of about 700° C., the content of SO2 and SO3, respectively, is equally large at equilibrium (depending on the sulphur content and oxygen content). In absence of the catalyst M, reaction (2) is slow at temperatures below 900° C. In flue-gas, normally the concentration of M is slow in relation to the gaseous phase reactants oxygen and sulphur trioxide. As a consequence, just a minor part of the sulphur dioxide has time to oxidise to sulphur trioxide before the flue-gas leaves the plant. This has been verified by measurements of the SO3-content in flue-gas at approx. 150° C., which indicates that only approx. 1-3% of the total sulphur oxide content in gaseous phase is present as SO3.
The problem of ensuring access to required amounts of reactant SO3 is satisfactory solved in WO 02/059526, which teaches addition of sulphurous compounds that instantly form SO3 for sulphating alkaline chloride in gaseous phase at the subject temperature range.
A non-solved problem is however that in some operational conditions the resulting sulphate is instable which may lead to a decreased technical effect.